Variable-ratio transformer



Patented Feb. 7, 1928.

UNITED STATES PATENT OFFICE.

FREDERICK F. BRAND, OF PITTSFIELD, MASSACHUSETTS, ASSIGNOR TO GENERAL ELECTRIC COMYANY, .A CORPORATION OF NEW YORK.

VARIABLE-RATIO TRANSFORMER.

Application filed July 6,

providing one of the windings with taps connected to a suitable switch so that the winding sections or groups of turns between the taps may be cut into or out of circuit to change the number of effective turns in the winding and thus the ratio of voltage transformation. While the tap switch is being ymoved from one tap connection to' another,

the circuit through the winding and the switch contacts is open so that it is necessary to have the load disconnected from the transformer while the change is being made to prevent serious arcing or sparking at the switch contacts.

It is usually inconvenient and often takes valuable time to open the primary circuit of the transformer while a tap connection in the secondary winding is being changed but it has been found that if this is not done therewill sometimes be suiiicient arcing at the switch contacts to injure these contacts even though the taps are in the secondary winding and the load has been disconnected from the transformer. This means that if the transformer is excited by current in the primary winding, there is some current in the secondary winding under some conditions even if the secondary or load circuit has been disconnected from the transformer. This current is very appreciable in high voltage transformers and is due to the .capa\ citance between the secondary winding leads and the bushings through which these leads are carried out of the transformer casing and also to the capacitance of the secondary winding itself. The capacity of the circuit through which the current iows makes the f current a leading one and such a current produces worse arcing or sparking than does either a lagging current vor one of unity power factor. The general object of the invention is to provide a simple and eicient arrangement to prevent arcing or sparking at the contacts of a tap switch connected to taps in the secondary winding of a transformer when the switch is operated with the transformer excited but disconnected from its secondary load circuit.

1927. Serial No. 203,824.

The invention will b'e better understood from the following description taken in connection with the accompanying drawing in which Fig. 1 shows an encased transformer provided with a tap switch, part of the casing being broken away; Fig. 2 shows diagrammaticallythe windings of the transformer, the secondary Winding being provided with one arrangement of taps connected to a tapswitch and provided with `means to prevent arcing or sparking at the fs\\"itcli"contacts, and Fig. 3 is similar to the l,view shown in Fig. 2 but with a different earrangement of taps.

Like reference characters indicate similar parts in the different figures of the drawing.

The transformer 10 shown in Fig. 1 is enclosed in a casing 11 containing a body of insulating oil 12 in which the transformer is immersed. The insulating oil extends to the level 13 in an expansion chamber 14 to permit expansion and contraction of the oil as its temperature changes. The leads 15 from the secondary winding 16 are carried out of the casing 11 through bushings 17 to terminals 18 for connection to an outside circuit. This secondary winding 16 is provided with taps connected to a tap switch 19 by means of which the ratio of voltage transformation of the transformer may be varied or adjusted as desired. The taps and tap switch may be arranged in various ways but it is usually desirable that the taps be located at some intermediate portion of the winding to protect them and the switch contacts from disturbances which may reach the winding through the outside circuit to which it may be connected.

One arrangement of the taps and switch which may be used is shown in Fig. 2. The taps, which are at one side of a break in the vsecondary circuit, are connected to the stationary contacts of a tap switch 19, the movable contact of the switch being connected to the secondary circuit at the other side of the break therein. Thus, the groups of winding turns between the taps nitty be made effective or ineffective by cutting them into or out of circuit in steps corresponding to the number and distribution of the taps.

It has been found that the capacitance of the secondary winding and the capacitance between the leads 15 and the bushings 17 which surround and insulate them where they pass out through the casing 11 are S1155:

tacts to reduce or prevent arcing or sparking at the contacts when the switches are opened or closed. A transformer tap v,switch is generally a multi-contact switcli,'however, and

would require several such by-passes to protect all its contacts.` The present invention provides such protection with a single by-pass, shown as a condenser 21.

The condenser 21 is of small capacity and is connected across the break in the sec` ondary circuit, one terminal 22 of the condenser being connected to the tapped portion of the winding and the other terminal 23 being connected to the secondary circuit at the other side of the break therein. lVhere this break is at an intermediate part of the winding as shown in Fig. 2, the second condenser terminal 23 should be connecte-d to the inner end of the untapped section of the winding. Now, when the switch 19 is operated to change `the voltage ratio of the transformer, any arcing or sparking at the switch contacts as they are o cned and closed will be negligible because o the shunt path provided by the condenser 21 through which any discharge of current may easily take place. This shunt path through the condenser 21 will, of course, include one or more groups of turns of the tapped portion of the winding except in theV case of the particular switch contact which maybe connected to the same point of the winding with the terininal, 22 of the condenser. There is some voltage induced in the groups of turns between taps and in order that the maximum value of the voltage between the condenser terminal 22 and any tap may be as small as possible, it isdesirable that the condenser terminal 22 be connected at or near the center of thc tapped portion of the winding.

Another arrangement of the taps and tap switch which may be used is shown in Fig. 3. ',Ihis is similar to the arrangement shown in Fig. 2 except that the contacts of the switch 19 arc connectcd to taps located at both sides of thc break in the secondary circuit and the, break must. therefore be atysome intermediate part 4of the secondary Winding. As shown, the switch is preferably arranged to cut groups of winding turns 1nto or out of circuit alternately from the two winding Sections .and thus to avoid shifting of the the center of its respective tapped portion' of the winding 16 so that the maximum volt,- age induced'in the turns between it and an'y tap will be as small as possible.

The principles of the invention have beenv explained by illustrating and describing two forms thereof but it will be apparent that various changes may be made without departing from the scope of the invention as defined in lthe appended claims.

lVhat I claim as new and desire to secure by Letters Patent of the United States,` is: 1. A transformer including primary and secondary windings, said secondary winding being provided with taps, there being a break in the secondary winding circuit adjacent said taps, a tap switch connected to said taps to vary the effective number of urns in said secondary winding, and a bypass connect-ed between-the tapped portion of said secondary winding and a point in said secondary winding circuit at the other side of said break therein.

2. A transformer including primary and secondary windings, said secondary winding being provided with taps, there being a .break in the secondary winding circuit adjacent said taps, a tap switch connected to said taps to -vary the effective number ofv turns in said secondary winding, and a by# pass connected between an intermediate point Iof the tapped portion of said secondary winding and a point in Said secondary winding circuit at the other side of said break therein.

3. A transformer including primary and secondary windings, saidsecondary winding vcomprising two sections with a break therebetween, each of said secondary winding sec-' tions beingprovided with taps adjacent said break, a tap switch connected to said taps to vary the effective number of turns in said secondary winding, and a by-pass connected between the tapped port-ions of said sccondary windingr sections.

Ll. A transformer including primary and secondary windings, said secondary winding i con'iln'ising two sections with a break therebetween. ,cach of said secondary winding sections being provided with taps adjacent said break. a tap switch connected to said taps to vary the effective number of turns in said .secondary winding, and a by-pass connected between inti-nmediate points of the tapped portions of said secondary/winding sections.

In witness whereof, I have hereunto set my hand this 2nd day of July,-1927.

- FREDERICK F. BRAND. 

